Lambda sensors or oxygen sensors are typically used to reduce emissions for vehicles with four-stroke engines by ensuring that the engines burn their fuel efficiently and cleanly. The sensor is normally configured to have one electrode in an exhaust gas pipe, whilst another electrode is in contact with the ambient air. The sensor operates by measuring the difference in oxygen concentration between the exhaust gas and the ambient air. The sensor typically outputs a voltage, and shows a step-type voltage characteristic around where the air-fuel ratio approaches the stoichiometric one. A stoichiometric air-fuel ratio corresponds to λ=1 and implies a good compromise between power, fuel economy and emissions for the four-stroke engine. λ<1 implies a higher voltage output and corresponds to excess of fuel and λ>1 implies a lower voltage output and corresponds to excess of air. When the engine is under low-load conditions (such as when accelerating very gently, or maintaining a constant speed), it is operating in a so called ‘closed-loop’ mode. This refers to a feedback loop between a control unit and the lambda sensor, in which the control unit adjusts the quantity of fuel and expects to see a resulting change in the response of the lambda sensor. This loop forces the engine to operate both slightly lean and slightly rich on successive loops, as it attempts to maintain a mostly stoichiometric ratio on average. When the engine is under high load (e.g. wide open throttle), the output of the oxygen sensor is ignored, and the control unit automatically enriches the mixture to protect the engine, as misfires under load are much more likely to cause damage. This is referred to an engine running in ‘open-loop’ mode. Any changes in the sensor output will be ignored in this state, since the fuel injection is controlled according to a predetermined so called fuel map. Closed-loop feedback-control of fuel varies the fuel output according to real-time sensor data rather than operating with a predetermined (open-loop) fuel map. Normally, lambda sensors only work effectively when heated to approximately 300° C., and consequently many one of today's sensors have integrated heating elements to ensure proper operation.
A disadvantage of four-stroke engines is that they are fairly heavy, and therefore they are not very suitable for applications that are intended to be carried by an operator. In fact, there are numerous applications in which two-stroke engines are preferred, such as chainsaws, trimmers, cut-off machines or blowers, etc, for which a high power-to-weight ratio is especially appreciated. Two-stroke engines are also advantageous in terms of their simple design and long service life. However, a major drawback with two-stroke engines is their poor emission performance. To meet the more stringent emission standards, there have been some attempts to regulate air-fuel ratio in two-stroke engines using a lambda sensor and a control unit in a feedback loop. However, the way λ is measured in an exhaust pipe in a four-stroke engine is not applicable to a two-stroke engine, since scavenging air will follow the exhaust gas resulting from the combusted air-fuel mixture in the combustion chamber into the exhaust gas pipe. Thus, λ at a corresponding point in an exhaust pipe will fluctuate and will not correspond to λ for the gases participating in combustion.
JP8144817 and JP8189386 (Yamaha) disclose a way of using a lambda sensor connected to a control unit in a two-stroke engine. To solve the problem with the scavenging air being pumped into the exhaust passage, a sub-exhaust passage in communication with the combustion chamber has been introduced, in which passage λ is detected. The sub-exhaust passage is provided with valves which are configured to close and open the passage in a manner such that it is closed before the scavenging air arrives. Thus, the detected λ will be near combustion λ. However, this solution is rather complicated and expensive, and inter alia requires additional valves and ducts in the cylinder. Further, there is a risk that said valves may clog with soot and/or oil, for example, which reduces functionality of the valves and thereby reduces the accuracy of the combustion λ measurement.
DE202006018582U1 (Dolmar) also discloses a way of using a lambda sensor connected to a control unit in a two-stroke engine. In analogy with the two Japanese documents above, this German document discloses a way to improve measuring of combustion λ but has located the lambda sensor in a piston ported space in the cylinder. The space opens up in the combustion chamber and is repeatedly opened and closed by the moving piston, such that it is closed before the scavenging air arrives in the combustion chamber. Thus, the detected λ will be near combustion λ. A disadvantage with this solution is that a desired combustion λ in a two-stroke engine is about 0.70 to 0.95. Due to the operation of today's lambda sensors, such low values are difficult to detect. Although, broadband lambda sensors are suitable for detecting λ-values within said range, they are far too expensive for this application. Also, a combustion λ detection in an encapsulated space that opens and closes to the combustion chamber does not really seem to be very accurate as the space will not be fully emptied after λ has been detected and, thus, the gases in which λ is detected will probably be gases resulting from earlier combustions. Further, there will probably gather oil in said space, which may cause the decisive portion of the lambda sensor to be at least partly covered in oil, which reduces the functionality of the lambda sensor.
U.S. Pat. No. 6,912,979 (Stihl) describes a way of adjusting λ in a crankcase in a range of approximately 0.2 to 0.6, and, thus, unlike what is proposed in the Japanese documents and the German document, λ is not detected in the exhaust gas of the air-fuel mixture participating in combustion. The U.S. document means that having this λ-range in the crankcase implies that λ in the combustion chamber may be adjusted to the range 0.70 to 0.95. However, such low values of λ, 0.2-0.6, are difficult to detect in a cost-effective way, as was discussed in above section.
It has proved beneficial for the two-stroke engines to adjust λ in the range 0.70-0.95 in the combustion chamber, which means that there is an excess of fuel. This range, however, as discussed in earlier sections is difficult to detect using a conventional lambda sensor. This means that even though the Japanese documents and the German document may have come close to detecting combustion λ, their solutions will be far too expensive to produce. Hence, there is a need to solve the problems above in order to meet the more stringent emission standards and improve performance of two-stroke engines.